The field of the invention generally relates to echo receiving systems and, more particularly, to apparatus for digitizing pulse radar returns.
As is well known in the art, marine pulse radars are used to detect the presence of objects which produce echo returns from transmitted pulses. Generally, when a transmitted pulse strikes a distant object, such as, for example, another ship or land, an echo is reflected back to the radar antenna which is coupled to the radar receiver. The elapsed time from transmission to receipt is proportional to the distance traveled. Accordingly, the input to the receiver for one transmitted pulse can be characterized as a train of echos or pulses from objects at different distances. Generally, the echos can be characterized into signals which are of interest and clutter which is not. For example, sea clutter is the result of reflections from the water and rain clutter is the result of reflections from moisture in the air. As is well known, it is important to be able to detect targets in the presence of clutter.
In one conventional type of radar system, the analog video from the radar receiver is coupled to a threshold circuit where the video is compared with one or more threshold voltages to digitize the video into two or more discrete levels. All but one of the levels are illuminated at some intensity or color on the display while the lowest level is not illuminated. Accordingly, when the analog video is above one or more of the thresholds, the presence of an echo is indicated and when it is below, the absence of an echo is indicated.
Prior art pulse marine radar systems have used adjustable thresholds so that the operator can increase or decrease the level above which the video is displayed. Also, prior art systems have provided the threshold with a sensitivity time control for use when clutter is present. More specifically, the threshold voltage has been generated by a decay circuit that provides a decreasing threshold voltage as time or range increases so that strong clutter from short range will be rejected approximately the same as weak clutter from long range. Further, adjustable fast time constant FTC high pass filters have been used to help suppress rain clutter.
The above-described prior art approaches for rejecting clutter provide only one threshold versus time or equivalently, one range function shape. However, a wide variety of conditions can occur at sea and the shape of the threshold voltage as a function of time or range may not be optimized for many of them. Stated differently, the change in mean level with range is so great at short ranges that conventional FTC filters have not effectively removed the mean from the video. Also, the peaks of clutter have a different relationship to the mean under various environmental conditions such that they are all not effectively matched by a single threshold waveform. As a consequence, it is common in prior art systems to have the threshold voltage either too low at one range such that clutter remains intense thereby obscuring targets in that region or unnecessarily high at another range such that targets which are suitably above the clutter are not detected in that region.